Thin-films have been widely used to fabricate various electronic, optical and magnetic structures because the processing of the thin-films may be precisely controlled, allowing the manufacture of complex components. These films are typically thermally grown or deposited from a vapor phase. Often the films are formed from metals, semiconductors or insulators, and must satisfy rigorous chemical, structural and electrical requirements. In addition, film composition and thickness must be strictly controlled to facilitate etching of sub-micron features. Hence, there is a great need to monitor thin-film structures during the manufacturing process to ensure they adhere to specified parameters.
Originally, the thickness of thin-film structures was measured using a stylus instrument. Use of a stylus, however, entailed contact with the thin-film surface, often resulting in damage which proved particularly unsuitable for use with soft materials. Ellipsometry was also employed to measure thin-film thicknesses to avoid the destructive contact of the stylus.
U.S. Pat. Nos. 5,293,214 and 5,291,269 to Ledger each discloses a device for measuring thin-film thickness employing reflectometry. These devices include a source directing light onto a condensing lens and subsequently through an aperture to illuminate a silicon wafer having a thin-film disposed on it. Light reflected from the thin-film is collected by a CCD array. The reflected light contains an image of an interference fringe pattern that is formed by constructive interference of light reflected from the physical boundaries within the wafer. The light collected by the CCD array is converted to a map of measured reflectance data by a digitizing circuit and a computer. The data is then compared to reference reflected data to generate a map of the thin-film layer thickness over a full aperture of the wafer.
The drawback with relying on ellipsometry or reflectometry is that it is inaccurate unless the optical properties of the film are known, e.g., absorbance and index of refraction. In addition to the aforementioned drawbacks, the optical measuring techniques of the prior art are unsuitable for measuring opaque thin-films. This results from the prior art techniques relying on the observance of light reflected from a subsurface thin-film substrate interface. Small angle X-ray scattering has, however, proved useful in measuring opaque thin-films due to the penetrability of the relatively short wavelength. Cowley and Ryan show that structural properties of thin films, such as thickness, density and smoothness, may be determined by analyzing the intensity of interference fringes formed by X-rays reflected from a thin-film structure at various angles. See R. A. Cowley and T. W. Ryan, "X-ray scattering studies of thin films and surfaces: thermal oxides on silicon", J. Phys. D, vol. 20 (1987). Specifically, Cowley and Ryan showed that the angular distance between the fringes is an accurate measure of the thin-film's thickness; the intensity of the fringes corresponds to thin-film density, relative to the density of the substrate; and the change of intensity with respect to the change of reflection angle corresponds to the smoothness of the thin-film/air interface and the thin-film/substrate interface.
FIG. 1 shows a prior art device similar to that used by Cowley and Ryan including a source 11 producing a bundle of X-rays 13 directed onto a planar monochromator 15. X-rays 17 reflected from the monochromator 15 are monochromatic and directed onto a thin-film 19 deposited on a substrate 21. An X-ray detector 23 is positioned to collect X-rays 25 reflected from the thin-film 19. In order to collect the rays reflected from the surface at various angles of incidence .theta., the substrate 21 is placed upon a stage 27 capable of pivoting about an axis 29 extending perpendicular to both the plane of the substrate and the direction of travel of the X-rays.
U.S. Pat. No. 5,003,569 to Okada et al. discloses a method and an apparatus using X-rays to determine a thickness of organic films. An organic film is irradiated with X-rays produced by a source, and a detector is positioned to collect X-rays reflecting from the sample's surface. The irradiation angle is continuously changed by moving the source of X-rays with respect to the organic film. The detector is moved accordingly to collect the X-rays reflected from the film at different irradiation angles. A drawback with the aforementioned X-ray scattering detection devices is that consecutive measurements must be made to obtain measurements at various angles of incidence, greatly increasing the time necessary to analyze the thin-film structure.
It is an object, therefore, of the present invention to decrease the time required to measure the structure of a thin-film layer by concurrently impinging x-rays on a thin-film surface at various angles and concurrently detecting X-rays reflected from the thin-film.